348 ELEMENTARY CHEMICAL MICROSCOPY 



precipitated by bichromate from neither acid, neutral nor am* 

 moniacal solutions. 



The strontium salt of the formula SrCr0 4 appears from 

 ammoniacal solution as exceedingly tiny yellow globulites or 

 dumb-bell-like aggregates; it is dimorphic, being either ortho- 

 rhombic or monoclinic. If the former, it is isomorphous with 

 the barium salt. 



When this test is used, acidify the dilute drop with acetic 

 acid, then add the fragment of bichromate. Do not stir, and 

 avoid rubbing the glass with rod or wire. Barium chromate 

 separates at once if present. After several minutes decant if 

 a precipitate has formed. To the decanted solution or clear 

 drop add a small drop of ammonium hydroxide and examine 

 the preparation for dumb-bells of strontium chromate. 



If both barium and strontium are believed to be present it 

 is best to warm the preparation to cause as complete a precipi- 

 tation of barium chromate as possible before adding the am- 

 monium hydroxide, but care must be taken to avoid unduly 

 concentrating the drop. It is also usually better to allow the 

 ammonium hydroxide to flow into the drop from one side rather 

 than add it directly to the middle of the drop. 



Normal potassium chromate produces, with barium salts, a 

 precipitate similar to that obtained with dichromate, but is not 

 to be recommended as a reagent because of its property of also 

 precipitating strontium compounds in acid solution. 



Ordinarily the precipitate of barium chromate is mostly 

 amorphous in appearance. Here and there, however, will be 

 found areas where there are recognizable crystals. A high 

 power is always required for the recognition of the form of the 

 crystals, hence the drop to be studied must be spread out quite 

 thin. 



Free mineral acids interfere with the test. 



In addition to barium and strontium, it must be remembered 

 that dichromate will also yield crystalline precipitates with 

 silver, lead, mercury and thallium, but in these cases nitric acid 

 may be present. 



